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Message-ID: <CALJ9ZPPv6Yj6FrpLY9u0G_4y1Th8BzjM+eTsYJAhRViQuaSfRA@mail.gmail.com>
Date: Thu, 9 May 2024 14:04:31 -0700
From: Yabin Cui <yabinc@...gle.com>
To: Namhyung Kim <namhyung@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
Arnaldo Carvalho de Melo <acme@...nel.org>, Mark Rutland <mark.rutland@....com>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>,
Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>,
linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
bpf@...r.kernel.org
Subject: Re: [PATCH v2] perf/core: Save raw sample data conditionally based on
sample type
Thanks for reviewing the patch! I will update a v3 patch based on the comments.
On Thu, May 9, 2024 at 12:55 PM Namhyung Kim <namhyung@...nel.org> wrote:
>
> Another thought.
>
> On Thu, May 9, 2024 at 12:45 PM Namhyung Kim <namhyung@...nel.org> wrote:
> >
> > Hello,
> >
> > On Wed, May 1, 2024 at 5:06 PM Yabin Cui <yabinc@...gle.com> wrote:
> > >
> > > Currently, space for raw sample data is always allocated within sample
> > > records for both BPF output and tracepoint events. This leads to unused
> > > space in sample records when raw sample data is not requested.
> >
> > Oh, I thought it was ok because even if it sets _RAW bit in the
> > data->sample_flags unconditionally, perf_prepare_sample() and
> > perf_output_sample() checks the original event's attr->sample_type
> > so the raw data won't be recorded.
> >
> > But I've realized that it increased data->dyn_size already. :(
> > Which means the sample would have garbage at the end.
> >
> > I need to check if there are other places that made the same
> > mistake for other sample types.
> >
> > >
> > > This patch checks sample type of an event before saving raw sample data
> > > in both BPF output and tracepoint event handling logic. Raw sample data
> > > will only be saved if explicitly requested, reducing overhead when it
> > > is not needed.
> > >
> > > Fixes: 0a9081cf0a11 ("perf/core: Add perf_sample_save_raw_data() helper")
> > > Signed-off-by: Yabin Cui <yabinc@...gle.com>
> >
> > Acked-by: Namhyung Kim <namhyung@...nel.org>
> >
> > Thanks,
> > Namhyung
> >
> > > ---
> [SNIP]
> > > @@ -10180,13 +10181,18 @@ static void __perf_tp_event_target_task(u64 count, void *record,
> > > /* Cannot deliver synchronous signal to other task. */
> > > if (event->attr.sigtrap)
> > > return;
> > > - if (perf_tp_event_match(event, data, regs))
> > > + if (perf_tp_event_match(event, raw, regs)) {
> > > + perf_sample_data_init(data, 0, 0);
> > > + if (event->attr.sample_type & PERF_SAMPLE_RAW)
> > > + perf_sample_save_raw_data(data, raw);
>
> Hmm.. to prevent future mistakes, maybe we can move the
> check into the perf_sample_save_raw_data() itself.
>
> And it'd be great if you could do the same for callchain
> and brstack too. :)
>
> Thanks,
> Namhyung
>
>
> > > perf_swevent_event(event, count, data, regs);
> > > + }
> > > }
> > >
> > > static void perf_tp_event_target_task(u64 count, void *record,
> > > struct pt_regs *regs,
> > > struct perf_sample_data *data,
> > > + struct perf_raw_record *raw,
> > > struct perf_event_context *ctx)
> > > {
> > > unsigned int cpu = smp_processor_id();
> > > @@ -10194,15 +10200,15 @@ static void perf_tp_event_target_task(u64 count, void *record,
> > > struct perf_event *event, *sibling;
> > >
> > > perf_event_groups_for_cpu_pmu(event, &ctx->pinned_groups, cpu, pmu) {
> > > - __perf_tp_event_target_task(count, record, regs, data, event);
> > > + __perf_tp_event_target_task(count, record, regs, data, raw, event);
> > > for_each_sibling_event(sibling, event)
> > > - __perf_tp_event_target_task(count, record, regs, data, sibling);
> > > + __perf_tp_event_target_task(count, record, regs, data, raw, sibling);
> > > }
> > >
> > > perf_event_groups_for_cpu_pmu(event, &ctx->flexible_groups, cpu, pmu) {
> > > - __perf_tp_event_target_task(count, record, regs, data, event);
> > > + __perf_tp_event_target_task(count, record, regs, data, raw, event);
> > > for_each_sibling_event(sibling, event)
> > > - __perf_tp_event_target_task(count, record, regs, data, sibling);
> > > + __perf_tp_event_target_task(count, record, regs, data, raw, sibling);
> > > }
> > > }
> > >
> > > @@ -10220,15 +10226,10 @@ void perf_tp_event(u16 event_type, u64 count, void *record, int entry_size,
> > > },
> > > };
> > >
> > > - perf_sample_data_init(&data, 0, 0);
> > > - perf_sample_save_raw_data(&data, &raw);
> > > -
> > > perf_trace_buf_update(record, event_type);
> > >
> > > hlist_for_each_entry_rcu(event, head, hlist_entry) {
> > > - if (perf_tp_event_match(event, &data, regs)) {
> > > - perf_swevent_event(event, count, &data, regs);
> > > -
> > > + if (perf_tp_event_match(event, &raw, regs)) {
> > > /*
> > > * Here use the same on-stack perf_sample_data,
> > > * some members in data are event-specific and
> > > @@ -10238,7 +10239,9 @@ void perf_tp_event(u16 event_type, u64 count, void *record, int entry_size,
> > > * because data->sample_flags is set.
> > > */
> > > perf_sample_data_init(&data, 0, 0);
> > > - perf_sample_save_raw_data(&data, &raw);
> > > + if (event->attr.sample_type & PERF_SAMPLE_RAW)
> > > + perf_sample_save_raw_data(&data, &raw);
> > > + perf_swevent_event(event, count, &data, regs);
> > > }
> > > }
> > >
> > > @@ -10255,7 +10258,7 @@ void perf_tp_event(u16 event_type, u64 count, void *record, int entry_size,
> > > goto unlock;
> > >
> > > raw_spin_lock(&ctx->lock);
> > > - perf_tp_event_target_task(count, record, regs, &data, ctx);
> > > + perf_tp_event_target_task(count, record, regs, &data, &raw, ctx);
> > > raw_spin_unlock(&ctx->lock);
> > > unlock:
> > > rcu_read_unlock();
> > > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > > index 9dc605f08a23..4b3ff71b4c0a 100644
> > > --- a/kernel/trace/bpf_trace.c
> > > +++ b/kernel/trace/bpf_trace.c
> > > @@ -620,7 +620,8 @@ static const struct bpf_func_proto bpf_perf_event_read_value_proto = {
> > >
> > > static __always_inline u64
> > > __bpf_perf_event_output(struct pt_regs *regs, struct bpf_map *map,
> > > - u64 flags, struct perf_sample_data *sd)
> > > + u64 flags, struct perf_raw_record *raw,
> > > + struct perf_sample_data *sd)
> > > {
> > > struct bpf_array *array = container_of(map, struct bpf_array, map);
> > > unsigned int cpu = smp_processor_id();
> > > @@ -645,6 +646,9 @@ __bpf_perf_event_output(struct pt_regs *regs, struct bpf_map *map,
> > > if (unlikely(event->oncpu != cpu))
> > > return -EOPNOTSUPP;
> > >
> > > + if (event->attr.sample_type & PERF_SAMPLE_RAW)
> > > + perf_sample_save_raw_data(sd, raw);
> > > +
> > > return perf_event_output(event, sd, regs);
> > > }
> > >
> > > @@ -688,9 +692,8 @@ BPF_CALL_5(bpf_perf_event_output, struct pt_regs *, regs, struct bpf_map *, map,
> > > }
> > >
> > > perf_sample_data_init(sd, 0, 0);
> > > - perf_sample_save_raw_data(sd, &raw);
> > >
> > > - err = __bpf_perf_event_output(regs, map, flags, sd);
> > > + err = __bpf_perf_event_output(regs, map, flags, &raw, sd);
> > > out:
> > > this_cpu_dec(bpf_trace_nest_level);
> > > preempt_enable();
> > > @@ -749,9 +752,8 @@ u64 bpf_event_output(struct bpf_map *map, u64 flags, void *meta, u64 meta_size,
> > >
> > > perf_fetch_caller_regs(regs);
> > > perf_sample_data_init(sd, 0, 0);
> > > - perf_sample_save_raw_data(sd, &raw);
> > >
> > > - ret = __bpf_perf_event_output(regs, map, flags, sd);
> > > + ret = __bpf_perf_event_output(regs, map, flags, &raw, sd);
> > > out:
> > > this_cpu_dec(bpf_event_output_nest_level);
> > > preempt_enable();
> > > --
> > > 2.45.0.rc0.197.gbae5840b3b-goog
> > >
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